As is known in the art, a buck DC-to-DC converter is a voltage step down and current step up converter. A buck DC-to-DC converter has a power switching section and a low pass filter section. The power switching section reduces the DC component of the power supply voltage source and the filter section removes the high frequency harmonics created by the power switching section to generate the desired DC output voltage level.
The power switching section has a first switch with a first terminal connected to one terminal of a power supply voltage source. The power supply voltage source may be a battery or the rectified AC power mains. The second terminal of the first switch is connected to a filter section of the buck DC-to-DC converter. A second switch in the power switching section has a first terminal connected to a ground reference voltage terminal. The second terminal of the second switch is connected to the second terminal of the first switch and the filter section of the buck DC-to-DC converter. The first and second switches each have a control terminal that is connected to control circuitry that determines the switching frequency and duration of the activations of the first and second switches based on a feedback signal from an output of the buck DC-to-DC converter.
The input of the filter section is a first terminal of an inductor and the second terminal of the inductor is connected to a first terminal of a filter capacitor. The second terminal of the filter capacitor is connected to the ground reference voltage terminal. The output of the buck DC-to-DC converter is the common connection of the second terminal of the inductor and the first terminal of the filter capacitor. A sense circuit is commonly applied to the output terminal of the buck DC-to-DC converter to provide the feedback signal for the control circuitry.
The buck DC-to-DC converter operates in a continuous, synchronous, or pulse width modulated mode for higher current or heavily loaded operation. The first and second switches are activated and deactivated at a fixed frequency and the period between each activation and deactivation is determined by comparing the feedback signal with a desired reference signal to create the desired output voltage. When the buck DC-to-DC converter operates in a discontinuous, asynchronous or pulse frequency modulated mode for low current or lightly loaded operation, the switches do not supply the current from the power supply voltage source on each cycle and the current then supplied during the commutation mode where current is provided from the collapsing field of the inductor. Often the discontinuous mode is used in portable electronics such as smart cellular telephones, tablet computers, digital readers, etc. as a “sleep mode”. The only current required by the system in these applications is monitoring current for system maintenance (i.e. system clocking and timers, cellular network monitoring, wireless network monitoring).
In the pulse frequency modulation mode, the buck DC-to-DC converter turns on the first switch to apply the power supply voltage source to the inductor when the output voltage falls below a reference voltage. The first switch is then turned off when the current in the coil reaches a threshold value (sleep current limit). The second switch is turned on when the first switch is turned off. The second switch is then turned off when the current in the coil is fully discharged. The pulse frequency modulation mode is not typically used for large currents as the current limit is normally set low to maximize efficiency.
Buck DC-to-DC converter converters operate in the pulse frequency modulation mode have serious problems with noise coupling when operating a high current levels. Further, when the second switch is open, there is no path from the filter section for negative currents resulting from overvoltage situations at the output of the buck DC-to-DC converter.